1. Field of the Invention
The invention relates to a method of manufacturing X-ray resist based on copolymers of methacrylonitrile and methacrylic acid.
2. Discussion of the Background
Resists are used in manufacturing semiconductor elements. Their use is based, in general, on the fact that zones of differing solubility are developed on relatively thin polymer layers upon irradiation with energetic radiation, such as X-rays. Using this technique, high precision can be obtained on a submicron scale. This phenomenon can be exploited to either increase solubility under the action of specific solvents (positive resist) or reduced solubility (negative resist). A reduction of the solubility of the polymer material may be based primarily on cross-linking reactions.
Copolymers of methacrylic acid and methacrylonitrile are known as materials for electronbeam positive resist (see European OS No. 64 864). Such resists have high sensitivity and high plasma etching resistance. To produce them, a supporting substrate, which as a rule is a surface-oxidized silicon wafer, is coated with a polymer solution of the resist material to form a resist film about 0.7 micron thick. The resist film is then tempered in air (at 30-40% humidity) at 160.degree. C. for 1/2 hr. This is followed by exposure to the electron beam treatment and development by an organic solvent; methyl cellosolve (2-methoxyethanol). A refinement of this is presented in European OS No. 81 633, where the coating comprised of the methacrylic acid/methacrylonitrile copolymer in an organic solvent-diglyme, (1,1'-oxybis(2-methoxyethane) is produced by centrifugal coating (spin coating). The first temperature regime should be at 120.degree.-140.degree. C. for 10-30 min. The second at 140.degree.-150.degree. C. for another 10-30 min. The irradiation is with UV light. As developers, methanol, ethanol, aqueous ethanol, and aqueous acetone may be used. For highly resolved polymer patterns, aqueous ethanol is recommended.
In various other publications, copolymers of methacrylic acid with methacrylonitrile, generally together with methacrylic acid esters (such as methyl methacrylate) or with isobutylene are recommended as resists for energetic radiation (see Japanese OSs No. 78-81,114, 81-544,434, 81-53,114, 79-88,117, 77-31,029 and 78-162,297). The particular suitability of copolymers made of methacrylic acid and methacrylonitrile for forming dimethylglutarimide units is described in U.S. Pat. No. 3,964,908. The high resolution, high glass transition temperature and good heat stability of resists based on this copolymer are noted in that patent document. See also Hiraoka H., Welsh, L. W., and Bargon, J., 1981, J. Electrochem. Soc., May:1065.
In many respects, polymers based on methacrylic acid/methacrylonitrile copolymers meet the technical requirements for a polymeric substrate (film material) for resist purposes. The technical requirements applicable when this class of polymers is employed for X-ray lithography are, however, in some respects different. The procedure is generally based on the technology described above, but it has been found that the temperature program analogous to European OS No. 81 633 does not lead to optimal results. Further, the development of the resist mask has been found lacking and in need of improvement.
There is therefore a strongly felt need for a methacrylic acid/methacrylonitrile copolymer-based resist material which does not suffer the drawbacks suffered by existing technology.